Many patients have respiratory problems, and it often is advantageous for a doctor to be able to ascertain the maximum peak rate of flow of a single forced expiration. It is further desirable to have a small instrument readily used by the patient himself to measure the peak flow. With a peak flow meter it is possible for the patient to ascertain if his peak flow rate remains at a more or less constant level, or if it deteriorates over time, or if it improves.
Instruments for such use, especially by patients, have been developed and are available commercially. However, the instruments now on the market are not fully satisfactory. They often have a member extending through a slot in a cylindrical body and blown along the body by a patient's exhalation into the cylindrical body. Such type of construction allows for entry of contaminants into the body, some of which leave residue that impairs movement of the indicator member along the slot. Most such meters have compressed scales which are difficult to read.
It is desirable that upon initial testing by the doctor a marker be applied to the meter indicating desirable range for the particular patient, as well as an improved range and a poor or deteriorating range. Such markers as known today are generally not satisfactory in that they are not capable of representing a percentage above or below the desirable range, since the desirable range for a given patient might be quite different from that of another patient. All that can be indicated is a fixed number above or below the desirable range, but there will be a different percentage of the desirable range depending upon where the desirable range is located on the meter.
In accordance with known meters, as indicated above, having a slot in the body through which the indicator moves, an opportunity is presented for a patient accidentally to block movement of the indicator with a misplaced finger. In the usual case the indicator is moved by a piston within the body, and as the piston travels along the body a progressively greater amount of air is expelled through the slot as the piston moves from rest position. Accordingly, the piston travels to a point at which the venting of air from the body balances a spring restraining the piston. Upon the cessation of blowing, the piston is returned to rest position by the spring, with the indicator remaining in the position reached until manually returned to rest position. The indicator must be of a relatively lightweight construction. If it is heavy, it will have inertia, and even though the piston stops, the indicator will continue to move according to its inertia, and thus introduce false readings. However, if the indicator is of extremely light weight and is exposed exteriorly of the body through the slot in which it moves, then it must be relatively fragile and susceptible to damage.